A conventional semiconductor fabrication plant typically includes multiple fabrication areas or bays interconnected by a path, such as a conveyor belt. Each bay generally includes the requisite fabrication tools (interconnected by a subpath) to process semiconductor wafers for a particular purpose, such as photolithography, chemical-mechanical polishing, or chemical vapor deposition. Material stockers or stocking tools generally are located about the plant and store semiconductor wafers waiting to be processed. The wafers are typically stored in containers, such as cassettes, each of which can hold up to 25 wafers. The cassettes are then stored in carriers that facilitate movement throughout the plant.
The semiconductor fabrication plant, including the bays, material stockers and the interconnecting path, typically operates under control of a distributed computer system running a factory management program. In this environment, the automated material handling system (AMHS) conceptually includes the cassettes, the transportation system (e.g., paths) and control system (e.g., the distributed computer system). The AMHS also typically includes empty carriers management system as well as a separate test wafer management system.
A wafer is subjected to a photolithography process of some type and usually involves applying a layer of photoresist material, such as silicon dioxide, over the surface of a wafer using a coating machine. The wafer is then moved to an exposure tool, such as a photolithography stepper, that exposes the photoresist layer to a patterned light source. The light source is patterned using a mask structure or a reticle. The reticle contains clear and opaque features that generally define the pattern to be created in the photoresist layer. The exposed photoresist is then developed and regions of the photoresist are dissolved leaving a pattern on the photoresist layer. The exposed portions of the underlying wafer are then subjected to further processing.
Depending on the type of IC device being manufactured, the wafer may be subjected to the photolithography process several times as layers are formed successively over prior layers to ultimately form the semiconductor device. To perform the various photolithography processes, a semiconductor plant has a photolithography processing area that occupies a substantial amount of floor space and involves a high level of capital investment to maintain. The photolithography area usually includes a number of steppers that utilize an entire cataloged library of reticles. The number of reticles that need to be readily available can easily exceed one thousand, due to the number of different products that can be manufactured in one facility, with each reticle having a replacement cost of about $15K. The reticles are usually stored in a reticle storage system, centrally located within the photolithography area, and are cataloged by reticle identification number. The reticle is then transported via a conveyor system to the particular stepper in need of a certain reticle. One of the problems with managing reticles is that they are very delicate structures and can be damaged easily by excessive handling. They also need to be routinely inspected to ensure that they are still viable for use in making the intended product.
Cycle times for the photolithography processing areas have increased due to the wafer processing system's limited resources that are available to manage all of the options available on the processing line. One of the problems with the current reticle management systems is the substantial manual intervention required in managing the finite number of reticles in inventory, the limited number of the duplicate reticles available and the finite number of pods that move the reticles around the photolithography area. In addition, operators on the line must manually coordinate any changes to the reticle flow or storage plan. This approach has led to delays in the wafer processing system and has caused inefficiencies in manufacturing. Further, midstream changes in production are relatively difficult and slow to respond to due to the de-centralized management of reticles and reticle storage locations.
Many of the problems faced in managing reticles also apply to the management, handling and use of solder bump masks. A solder bump mask is another type of mask used in the manufacture of flip chips that is delicate and needs to be handled carefully. In the solder interconnect process, typically known as the controlled-collapse chip connection (C4) process, a pattern of solder bumps is deposited on a wettable conductive terminal of a flip chip. The solder bump pattern is then coupled to a substrate and reflowed to create an electrical and mechanical connection from the chip to the substrate. The solder bumps are typically formed by evaporating lead (Pb) through openings in a molybdenum mask that is clamped to the wafer. A laser is used to form many holes corresponding to the individual chip contacts on the wafer to form the mask. Solder bump masks tend to wear down more quickly than reticles due to the frequency of use and the cleanings with harsh chemicals. To ensure consistency in precision in the formation of the solder bump pattern, solder bump masks are discarded regularly. However, most solder bump masks are discarded prematurely since a closer inspection would indicate that the bump masks are still suitable for use, hence an increased cost in the manufacture of flip chip devices in not using a viable chip processing component.